Freefall forming of bulk metallic glass feedstock and sheet material

ABSTRACT

The disclosure is directed to freefall methods and apparatuses for preparation of amorphous BMG feedstock and sheet material. In certain aspects, the disclosure relates to methods and apparatuses for contactless formation of BMG feedstock and sheet material via a drop-tower. In certain embodiments, the methods comprise releasing droplets of molten amorphous alloy into a cooled, pressurized chamber of a drop-tower, wherein the droplets traverse the chamber through freefall to thereby form BMG feedstock or sheet material.

The application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 62/025,703, entitled “Freefall Forming of Bulk Metallic Glass Feedstock and Sheet Material,” filed on Jul. 17, 2014, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The disclosure is directed to methods of forming bulk metallic glass feedstock and sheets.

BACKGROUND

Amorphous alloys have a combination of high strength, elasticity, corrosion resistance and processability from the molten state. Amorphous alloys are generally processed and formed by cooling a molten alloy from above the melting temperature of the crystalline phase (or the thermodynamic melting temperature) to below the “glass transition temperature” of the amorphous phase at “sufficiently fast” cooling rates, such that the nucleation and growth of alloy crystals is avoided. As such, the processing methods for amorphous alloys have always been concerned with quantifying the “sufficiently fast cooling rate,” which is also referred to as “critical cooling rate”, to ensure formation of the amorphous phase. However, even with application of sufficiently fast cooling rates, problems may sometimes be encountered in preparation of amorphous feedstocks.

Conventional processes have not been suitable for forming amorphous alloys, and special casting processes such as melt spinning and planar flow casting are often used. For crystalline alloys having fast crystallization kinetics, extremely short times (on the order of 10⁻³ seconds or less) for heat extraction from the molten alloy are used to bypass crystallization. Such amorphous alloys are capable of forming only very thin amorphous foils and ribbons (order of 25 microns in thickness). Conventional processes can also present difficulties when bulk metallic glasses, or “BMGs,” are used, even though BMGs have longer lifetimes allowing them to be processed on longer time scales.

However, difficulties are still encountered during casting and molding of amorphous foils as well as bulk metallic glasses (“BMGs”). As such, there is still a need for improved methods of forming amorphous feedstock and sheet materials of BMGs.

SUMMARY

Described herein are freefall methods and apparatuses for preparation of amorphous BMG feedstock and sheet material.

In accordance with certain aspects, the disclosure relates to methods and apparatuses for contactless formation of BMG feedstock and sheet material via a drop-tower. In certain embodiments, the methods comprise releasing droplets of molten amorphous alloy into a cooled, pressurized chamber of a drop-tower, wherein the droplets traverse the chamber through freefall to thereby form BMG feedstock or sheet material.

BRIEF DESCRIPTION OF FIGURES

Although the following figures and description illustrate specific embodiments and examples, the skilled artisan will appreciate that various changes and modifications may be made without departing from the spirit and scope of the disclosure.

FIG. 1 shows an exemplary freefall system of the disclosure.

FIG. 2 provides a temperature-viscosity diagram of an exemplary bulk solidifying amorphous alloy.

FIG. 3 provides a schematic of a time-temperature-transformation (T) diagram for an exemplary bulk solidifying amorphous alloy.

DETAILED DESCRIPTION

The disclosure is directed to freefall methods and apparatuses for preparation of amorphous BMG feedstock and sheet material. In certain aspects, the disclosure relates to methods and apparatuses for contactless formation of BMG feedstock and sheet material via a drop-tower. In certain embodiments, the methods comprise releasing droplets of molten amorphous alloy into a cooled, pressurized chamber of a drop-tower, wherein the droplets traverse the chamber through freefall to thereby form BMG feedstock or sheet material. Without intending to be limited by theory, the methods and apparatuses of the disclosure allow for preparation of amorphous BMG feedstock and sheet material without use of a tool which must come into contact with the material for cooling. As such, there is no nucleation point for crystallization, thereby minimizing and reducing potential for crystallization of the amorphous BMG feedstock and sheet material.

Amorphous alloys differ from conventional crystalline alloys in that their atomic structure lack the typical long range ordered patterns of the atomic structure of conventional crystalline alloys. BMGs are amorphous alloys that typically have critical cooling rates as low as a few ° C./second, which allows the processing and forming of much larger bulk amorphous objects. In various aspects, BMGs have a critical rod diameter of at least 1 mm. As used herein, the “critical rod diameter” is the largest rod diameter in which the amorphous phase can be formed when processed by the method of water quenching a quartz tube with 0.5 mm thick wall containing a molten alloy. As used herein, “amorphous alloy” and “metallic glass” are used interchangeably.

BMGs solidify and cool at relatively slow rates, and they retain the amorphous, non-crystalline (i.e., glassy) state at room temperature. However, if the cooling rate is not sufficiently high or nucleation sources are present, crystals may form inside the alloy during cooling, so that the benefits of the amorphous state can be lost. For example, partial crystallization of parts intended to be formed of BMG materials due to either slow cooling or impurities in the raw alloy material results in loss of amorphous character, and hence failure to form a BMG. As such, there is a need to develop methods for forming amorphous BMG feedstock and sheet material having reduced or no crystallinity.

BMGs can be inherently difficult to process, mold and solidify in the amorphous state before crystallization begins. One additional factor that can accelerate or exacerbate onset of crystallization is the grain structure of materials that may come in contact with them during processing and molding. Without intending to be limited by theory, the grain structure of such materials may act as a nucleation point for BMG crystallization. This may be more significant for certain types of amorphous alloys as compared to others, e.g., platinum-based alloys. For instance, with Pt-based alloys, the onset of nucleation quickly spreads throughout the rest of the alloy, quickly rendering a solidified part or feedstock almost entirely crystalline.

In certain aspects, the disclosure relates to methods and apparatuses for contactless formation of BMG feedstock and sheet material via a drop-tower. In certain embodiments, the methods comprise releasing droplets of molten amorphous alloy into a cooled, pressurized chamber of a drop-tower, wherein the droplets traverse the chamber through freefall to thereby form BMG feedstock or sheet material. Without intending to be limited by theory, the methods and apparatuses of the disclosure allow for preparation of amorphous BMG feedstock and sheet material without use of a tool which must come into contact with the material for cooling. As such, there is no nucleation point for crystallization, thereby minimizing and reducing potential for crystallization of the amorphous BMG feedstock and sheet material.

With reference to FIG. 1, by way of example, an exemplary freefall system 100 is shown, wherein molten amorphous alloy 102 may be formed in a crucible 104 via induction coils, RF heater, or any other known manner (not shown). Droplets 108 of molten amorphous alloy are released from a nozzle/valve 106 or other suitable mechanism into a cooled, pressurized chamber 110 of drop-tower 112. The chamber 110 of drop-tower 112 is controllably cooled and pressurized via an inert gas quenchant 114 and related temperature/pressure controls 116. The droplets 108 of molten amorphous alloy traverse the chamber 110 through freefall, and are cooled via application of a pressurized inert gas quenchant 114. In this way, the inert gas is used to pressurize the atmosphere inside the chamber 110 and to cool and shape the droplets 108 of molten amorphous alloy, so as to form amorphous BMG feedstock/sheets 118. During the freefall, the droplets 108 of molten amorphous alloy are rapidly cooled and quenched by the controlled atmosphere of inert gas within the chamber.

As mentioned above, the methods and apparatuses of the disclosure are particularly suited for use in connection with certain molten amorphous alloys such as those prone to quick nucleation and crystallization. While the disclosure is not so limited and can be used in connection with any molten amorphous alloy as discussed herein, in certain aspects the methods and apparatuses are suited for use in connection with platinum-based alloys (i.e. an alloy in which platinum is the primary element). In certain embodiments, the methods and apparatuses of the disclosure may be used to prepare platinum-based alloy feedstocks and sheet materials, such as Pt—Cu—Ni—Al alloy feedstocks and sheet materials.

Any suitable method of forming the molten amorphous alloy may be used in connection with the disclosure, including but not limited to induction coils or RF heaters as described in connection with FIG. 1. Further, any suitable crucible or melting apparatus may be utilized. The melting apparatus may be configured to interface with the drop-tower in any suitable manner, so as to provide for fluid communication of the droplets of molten amorphous alloy to the pressurized chamber of the drop-tower. In certain embodiments, the droplets of the molten amorphous alloy are provided in a controlled manner via, e.g., a controlled valve or similar configuration.

The drop-tower may be formed from any suitable material, and may be sized and shaped in any desired manner. By way of example, the drop-tower may be sized and shaped depending on the cooling parameters of the amorphous alloy. In certain embodiments, the drop-tower may range from, e.g., 0.5 m to 5 m or more in length. Further, the drop-tower may be configured to withstand the desired operating temperatures and pressures required to provide the rapid cooling of the methods of the disclosure so as to maintain an amorphous alloy, i.e., those required to provide amorphous BMG feedstock and sheet material.

Any suitable inert gas may be used in connection with the disclosure, including but not limited to, argon, helium, and combinations thereof. However, the disclosure is not so limited. The chamber of the drop-tower may be operated at any suitable pressure so as to achieve suitable cooling to maintain an amorphous alloy. In this regard, the pressure may range from atmospheric pressure to pressures slightly above atmospheric pressure, to significantly above atmospheric pressure. As will be understood, pressures will impact cooling rates as well as shaping of the cooling BMG feedstock/sheet material. Selection of droplet parameters, drop-tower configuration and operating parameters will depend, in part, on amorphous alloy characteristics.

FIG. 2 (obtained from U.S. Pat. No. 7,575,040) shows a viscosity-temperature graph of an exemplary bulk solidifying amorphous alloy, from an exemplary series of Zr—Ti—Ni—Cu—Be alloys manufactured by Liquidmetal Technology. It should be noted that there is no clear liquid/solid transformation for a bulk solidifying amorphous metal during the formation of an amorphous solid. The molten alloy becomes more and more viscous with increasing undercooling until it approaches solid form around the glass transition temperature. Accordingly, the temperature of solidification front for bulk solidifying amorphous alloys can be around glass transition temperature, where the alloy will practically act as a solid for the purposes of pulling out the quenched amorphous sheet product.

FIG. 3 (obtained from U.S. Pat. No. 7,575,040) shows the time-temperature-transformation (TTT) cooling curve of an exemplary bulk solidifying amorphous alloy, or TTT diagram. Bulk-solidifying amorphous metals do not experience a liquid/solid crystallization transformation upon cooling, as with conventional metals. Instead, the highly fluid, non-crystalline form of the metal found at high temperatures (near a “melting temperature” Tm) becomes more viscous as the temperature is reduced (near to the glass transition temperature Tg), eventually taking on the outward physical properties of a conventional solid.

Even though there is no liquid/crystallization transformation for a bulk solidifying amorphous metal, a melting temperature Tm may be defined as the thermodynamic liquidus temperature of the corresponding crystalline phase. Under this regime, the viscosity of bulk-solidifying amorphous alloys at the melting temperature could lie in the range of about 0.1 poise to about 10,000 poise, and even sometimes under 0.01 poise. A lower viscosity at the “melting temperature” would provide faster and complete filling of intricate portions of the shell/mold with a bulk solidifying amorphous metal for forming the BMG parts. Furthermore, the cooling rate of the molten metal to form a BMG part has to such that the time-temperature profile during cooling does not traverse through the nose-shaped region bounding the crystallized region in the TTT diagram of FIG. 3. In FIG. 3, Tnose is the critical crystallization temperature Tx where crystallization is most rapid and occurs in the shortest time scale.

The supercooled liquid region, the temperature region between Tg and Tx is a manifestation of the stability against crystallization of bulk solidification alloys. In this temperature region the bulk solidifying alloy can exist as a high viscous liquid. The viscosity of the bulk solidifying alloy in the supercooled liquid region can vary between 10¹² Pa s at the glass transition temperature down to 10⁵ Pa s at the crystallization temperature, the high temperature limit of the supercooled liquid region. Liquids with such viscosities can undergo substantial plastic strain under an applied pressure. The embodiments herein make use of the large plastic formability in the supercooled liquid region as a forming and separating method.

Technically, the nose-shaped curve shown in the TTT diagram describes Tx as a function of temperature and time. Thus, regardless of the trajectory that one takes while heating or cooling a metal alloy, when one hits the TTT curve, one has reached Tx. In FIG. 3, Tx is shown as a dashed line as Tx can vary from close to Tm to close to Tg.

The schematic TTT diagram of FIG. 3 shows processing methods of die casting from at or above Tm to below Tg without the time-temperature trajectory (shown as (1) as an example trajectory) hitting the TTT curve. During die casting, the forming takes place substantially simultaneously with fast cooling to avoid the trajectory hitting the TTT curve. The processing methods for superplastic forming (SPF) from at or below Tg to below Tm without the time-temperature trajectory (shown as (2), (3) and (4) as example trajectories) hitting the TTT curve. In SPF, the amorphous BMG is reheated into the supercooled liquid region where the available processing window could be much larger than die casting, resulting in better controllability of the process. The SPF process does not require fast cooling to avoid crystallization during cooling. Also, as shown by example trajectories (2), (3) and (4), the SPF can be carried out with the highest temperature during SPF being above Tnose or below Tnose, up to about Tm. If one heats up a piece of amorphous alloy but manages to avoid hitting the TTT curve, you have heated “between Tg and Tm”, but one would have not reached Tx.

Typical differential scanning calorimeter (DSC) heating curves of bulk-solidifying amorphous alloys taken at a heating rate of 20 C/min describe, for the most part, a particular trajectory across the TTT data where one would likely see a Tg at a certain temperature, a Tx when the DSC heating ramp crosses the TTT crystallization onset, and eventually melting peaks when the same trajectory crosses the temperature range for melting. If one heats a bulk-solidifying amorphous alloy at a rapid heating rate as shown by the ramp up portion of trajectories (2), (3) and (4) in FIG. 3, then one could avoid the TTT curve entirely, and the DSC data would show a glass transition but no Tx upon heating. Another way to think about it is trajectories (2), (3) and (4) can fall anywhere in temperature between the nose of the TTT curve (and even above it) and the Tg line, as long as it does not hit the crystallization curve. That just means that the horizontal plateau in trajectories might get much shorter as one increases the processing temperature.

Any amorphous alloy in the art may be used in connection with the methods and apparatuses described herein.

The methods and apparatuses described herein can be applicable to any type of suitable amorphous alloy. Similarly, the amorphous alloy described herein as a constituent of a composition or article can be of any type. As recognized by those of skill in the art, amorphous alloys may be selected based on and may have a variety of potentially useful properties. In particular, amorphous alloys tend to be stronger than crystalline alloys of similar chemical composition.

The amorphous alloy can comprise multiple transition metal elements, such as at least two, at least three, at least four, or more, transitional metal elements. The amorphous alloy can also optionally comprise one or more nonmetal elements, such as one, at least two, at least three, at least four, or more, nonmetal elements. A transition metal element can be any of scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, mercury, rutherfordium, dubnium, seaborgium, bohrium, hassium, meitnerium, ununnilium, unununium, and ununbium. In one embodiment, a BMG containing a transition metal element can have at least one of Sc, Y, La, Ac, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, and Hg. Depending on the application, any suitable transitional metal elements, or their combinations, can be used.

Depending on the application, any suitable nonmetal elements, or their combinations, can be used. A nonmetal element can be any element that is found in Groups 13-17 in the Periodic Table. For example, a nonmetal element can be any one of F, Cl, Br, I, At, O, S, Se, Te, Po, N, P, As, Sb, Bi, C, Si, Ge, Sn, Pb, and B. Occasionally, a nonmetal element can also refer to certain metalloids (e.g., B, Si, Ge, As, Sb, Te, and Po) in Groups 13-17. In one embodiment, the nonmetal elements can include B, Si, C, P, or combinations thereof. Accordingly, for example, the alloy can comprise a boride, a carbide, or both.

The amorphous alloy can include any combination of the above elements in its chemical formula or chemical composition. The elements can be present at different weight or volume percentages. Alternatively, in one embodiment, the above-described percentages can be volume percentages, instead of weight percentages. Accordingly, an amorphous alloy can be zirconium-based, titanium-based, platinum-based, palladium-based, gold-based, silver-based, copper-based, iron-based, nickel-based, aluminum-based, molybdenum-based, and the like. The alloy can also be free of any of the aforementioned elements to suit a particular purpose. For example, in some embodiments, the alloy, or the composition including the alloy, can be substantially free of nickel, aluminum, titanium, beryllium, or combinations thereof. In one embodiment, the alloy or the composite is completely free of nickel, aluminum, titanium, beryllium, or combinations thereof.

Furthermore, the amorphous alloy can also be one of the exemplary compositions described in U.S. Patent Application Publication No. 2010/0300148 or 2013/0309121, the contents of which are herein incorporated by reference.

The amorphous alloys can also be ferrous alloys, such as (Fe,Ni,Co) based alloys. Examples of such compositions are disclosed in U.S. Pat. Nos. 6,325,868; 5,288,344; 5,368,659; 5,618,359; and 5,735,975, Inoue et al., Appl. Phys. Lett., Volume 71, p 464 (1997), Shen et al., Mater. Trans., JIM, Volume 42, p 2136 (2001), and Japanese Patent Application No. 200126277 (Pub. No. 2001303218 A). One exemplary composition is Fe₇₂Al₅Ga₂P₁₁C₆B₄. Another example is Fe₇₂Al₇Zr₁₀Mo₅W₂B₁₅. Another iron-based alloy system that can be used in the coating herein is disclosed in U.S. Patent Application Publication No. 2010/0084052, wherein the amorphous metal contains, for example, manganese (1 to 3 atomic %), yttrium (0.1 to 10 atomic %), and silicon (0.3 to 3.1 atomic %) in the range of composition given in parentheses; and that contains the following elements in the specified range of composition given in parentheses: chromium (15 to 20 atomic %), molybdenum (2 to 15 atomic %), tungsten (1 to 3 atomic %), boron (5 to 16 atomic %), carbon (3 to 16 atomic %), and the balance iron.

The afore described amorphous alloy systems can further include additional elements, such as additional transition metal elements, including Nb, Cr, V, and Co. The additional elements can be present at less than or equal to about 30 wt %, such as less than or equal to about 20 wt %, such as less than or equal to about 10 wt %, such as less than or equal to about 5 wt %. In one embodiment, the additional, optional element is at least one of cobalt, manganese, zirconium, tantalum, niobium, tungsten, yttrium, titanium, vanadium and hafnium to form carbides and further improve wear and corrosion resistance. Further optional elements may include phosphorous, germanium and arsenic, totaling up to about 2%, and preferably less than 1%, to reduce melting point. Otherwise incidental impurities should be less than about 2% and preferably 0.5%.

In some embodiments, a composition having an amorphous alloy can include a small amount of impurities. The impurity elements can be intentionally added to modify the properties of the composition, such as improving the mechanical properties (e.g., hardness, strength, fracture mechanism, etc.) and/or improving the corrosion resistance. Alternatively, the impurities can be present as inevitable, incidental impurities, such as those obtained as a byproduct of processing and manufacturing. The impurities can be less than or equal to about 10 wt %, such as about 5 wt %, such as about 2 wt %, such as about 1 wt %, such as about 0.5 wt %, such as about 0.1 wt %. In some embodiments, these percentages can be volume percentages instead of weight percentages. In one embodiment, the alloy sample/composition consists essentially of the amorphous alloy (with only a small incidental amount of impurities). In another embodiment, the composition includes the amorphous alloy (with no observable trace of impurities).

The methods herein can be valuable in the fabrication of electronic devices using a BMG-containing part. An electronic device herein can refer to any electronic device known in the art. For example, it can be a telephone, such as a mobile phone, and a land-line phone, or any communication device, such as a smart phone, including, for example an iPhone®, and an electronic email sending/receiving device. It can be a part of a display, such as a digital display, a TV monitor, an electronic-book reader, a portable web-browser (e.g., iPad®), and a computer monitor. It can also be an entertainment device, including a portable DVD player, conventional DVD player, Blue-Ray disk player, video game console, music player, such as a portable music player (e.g., iPod®), etc. It can also be a part of a device that provides control, such as controlling the streaming of images, videos, sounds (e.g., Apple TV®), or it can be a remote control for an electronic device. It can be a part of a computer or its accessories, such as the hard drive tower housing or casing, laptop housing, laptop keyboard, laptop track pad, desktop keyboard, mouse, and speaker. The article can also be applied to a device such as a watch or a clock.

All publications, patents, and patent applications cited in this Specification are hereby incorporated by reference in their entirety.

While this disclosure has been described with reference to specific embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof, without departing from the spirit and scope of the disclosure. In addition, modifications may be made to adapt the teachings of the disclosure to particular situations and materials, without departing from the essential scope thereof. Thus, the disclosure is not limited to the particular examples that are disclosed herein, but encompasses all embodiments falling within the scope of the appended claims. 

1. A method of forming a metallic glass comprising: releasing a droplet of a molten metallic glass-forming alloy from a point above a surface into a drop-tower chamber held at or above atmospheric pressure; and allowing the droplet to fall through the pressurized drop-tower chamber to form a metallic glass.
 2. The method of claim 1, comprising melting the metallic glass-forming alloy using a melting apparatus before the step of releasing the droplet.
 3. The method of claim 2, wherein the melting apparatus comprises an induction coil, an RF heater, or a crucible.
 4. The method of claim 3, wherein the melting apparatus comprises an induction coil.
 5. The method of claim 3, wherein melting apparatus comprises an RF heater.
 6. The method of claim 3, wherein melting apparatus comprises a crucible.
 7. The method of claim 2, wherein the melting apparatus is in fluid communication with the pressurized drop-tower chamber.
 8. The method of claim 1, wherein the chamber contains an inert gas.
 9. The method of claim 8, wherein the inert gas is selected from argon, helium, and a combination thereof.
 10. The method of claim 1, wherein the metallic glass-forming alloy is selected from a zirconium-based alloy, titanium-based alloy, platinum-based alloy, palladium-based alloy, gold-based alloy, silver-based alloy, copper-based alloy, iron-based alloy, nickel-based alloy, aluminum-based alloy, and molybdenum-based alloy.
 11. The method of claim 10, wherein the metallic glass-forming alloy is a platinum-based alloy.
 12. The method of claim 11, wherein the platinum-based alloy comprises Pt, Cu, Ni, and Al.
 13. An drop-tower apparatus comprising: a melting apparatus configured to melt a metallic glass-forming alloy; a droplet forming in fluid communication with the melting apparatus; a pressurized drop-tower chamber operably associated with the droplet forming component in a vertical orientation.
 14. The apparatus of claim 13, wherein the melting apparatus is selected from an induction coil, an RF heater, or a crucible.
 15. The method of claim 14, wherein the melting apparatus comprises an induction coil.
 16. The method of claim 3, wherein the melting apparatus comprises an RF heater.
 17. The method of claim 3, wherein the melting apparatus comprises a crucible.
 18. The apparatus of claim 13, wherein the droplet forming component comprises a nozzle.
 19. The method of claim 13, wherein the drop-tower chamber comprises an inert gas.
 20. A method of forming a metallic glass comprising: melting a metallic glass-forming alloy in a melting apparatus; releasing a droplet of the molten metallic glass-forming alloy from a point above a surface into a pressurized drop-tower chamber; and allowing the droplet to cool while falling through the chamber to form a metallic glass. 